The practice of drilling through a wellhead employed as a pressure vessel is well known in the art. It has been conventional practice to suspend each different casing size in the wellhead pressure vessel and, with each change in casing size, to cease drilling activity while the pressure controlling blowout preventers are disconnected and removed to allow a wellhead of proper bore size to be installed. After the wellhead is installed the blowout preventer is reinstalled and tested for pressure integrity before allowing the drilling operation to continue.
The well is without blowout pressure control and subject to blowout during such operational sequences in the current art. Any blowout can contribute to costly delays in a drilling program schedule.
While the blowout preventer pressure control apparatus is removed, high pressure and high temperature conditions in the well bore can become extreme, causing destructive gases and fluids to be expelled to the atmosphere. Accordingly, it is highly desirable to maintain pressure control of the well with pressure control devices at all times. The conventional prior art systems cannot provide such continuous pressure control.
Multi-bowl wellheads are also known where successive steps (450 shoulders) are formed within the wellhead to support more than one casing hanger and respective seal assemblies. Such wellheads have been one continuous assembly which accommodates two or more mandrel type casing hanger assemblies. Such wellheads also have been made up of two or more flanged wellhead spools into a single assembly.
In these prior art wellheads the internal diameter of the shoulders are determined by the load bearing requirements for the casing hangers and also the relationship to the passage of a drill bit through the reduced diameter at the shoulders in order to drill holes for casing to be subsequently set and cemented while suspended from that respective shoulder.
Where long casing strings and corresponding heavy weights are involved, the load bearing requirements of the support shoulder is frequently the limiting factor of the internal diameter at the shoulder, thus fixing the minimum I.D. of the wellhead. When the casing weight exceeds the load bearing requirements of a support shoulder, the addition of massive pins, driven support wedges, and multiple radial support screws have been used to supplement load shoulders as a means of extending the casing weight. These arrangements of course require penetration of the pressure vessel housing by these devices. The penetrations must be sealed by an elastomer packing which is much less suitable than metal-to-metal seals, or no penetration to begin with.
An additional advantage of the invention is the provision of metal-to-metal seals in all penetrations of the pressure housing. It is necessary to employ some threaded hold-down pins and alignment pins to actuate seal elements, to hold hangers in position, and to align support ring setting tools. While each of these types of pins have elastomer seals used during actuation, they are also provided with metal-to-metal final closure seals. Accordingly, each penetration of the pressure housing is sealed metal-to-metal providing a greater measure of pressure integrity.
As a further advantage, the invention provides structure to convert the drilling wellhead assembly into a well completion system. The conventional well completions employ successively smaller bores and higher pressure rated casing and tubing spools with corresponding size and pressure rated blowout preventers. As previously described, the well is left without the important pressure control during each change of the wellhead since the blowout preventer is removed during each installation of the successive wellhead.
The universal system of the present invention provides continuous attachment of the blowout preventer to the top flange connection of the wellhead and thereby provides maximum pressure control throughout the entire drilling and completion operation, including the drilling of holes for the several casing strings.
The universal wellhead structure and system of the present invention includes provision for receiving a multiple or dual tubing hanger assembly which consists of an external primary duplex bowl and a secondary mandrel sub-assembly which allows tubing installation with the multiple hanger in the blowout preventer. Thus the tubing is run in the multiple hanger, landed, and tested before the blowout preventer is removed. Thereafter a crossover adapter permits reduction of the Christmas tree flange to a smaller size, more convenient to remedial operations. The secondary mandrel insert hanger is the unique structure permitting the tubing to be removed and reinstalled for workover purposes.
There is much prior art pertaining to wellhead equipment generally and also pertaining to well drilling and completion operation. There is no presently known prior art which particularly pertains to the apparatus as herein disclosed. The Composite Catalogue of Oil Field Equipment and Services, 1984-1985, published by World Oil, Houston, Tex., illustrates such equipment generally.